Roller shades of this kind have been disclosed in patent documents U.S. Pat. No. 6,536,503 and U.S. Pat. No. 7,665,505. While being genuine efforts of eliminating operating cords and braking devices, the proposed devices have tended to be rather complicated and difficult in adapting to different sizes of shades with respect to heights and widths. It has also been proposed in patent application document WO 2010/089118 to provide a helically wound spring as an additional assisting device in cord and motor operated window coverings. However it was not recognised that this spring assist device would be suitable for roller shades that are devoid of additional braking devices, such as clutches and friction increasing means, or operating mechanisms, such as operating cords.
It has further been observed with the known roller shades that a stop that limits upward travel would be desirable, but was difficult to combine with known torque accumulating mechanisms, or to be adjusted.
Yet another concern has been the fine adjustment or readjustment of the known torque accumulating mechanisms for production tolerances or wear, which has been generally impossible to achieve.